The discovery of the nucleus Flashcards Preview

J J Thompson’s original model of atom was ‘plum pudding’, where negative electrons were dotted in the atom like the currents in a ‘current bun’, and the positive charge was spread throughout the atom like a positive dough.

2

What was done in Rutherfords α scattering experiment?

A source emitting a narrow beam of α particles, all with the same KE.

Was directed at a thin layer of gold foil.

Inside an evacuated container.

A microscope was used to observe the pinpoints of light emitted when an α particle hit the fluorescent screen.

3

Why was the chamber evacuated?

To prevent α particles being absorbed by air molecules.

4

Why was the foil thin?

To prevent α particles being scattered more than once.

5

Why do the α particles need the same speed?

Slower α particles are deflected more than faster α particles on the same initial path.

6

What happened to the α particles approaching the nucleus?

Most passed straight through with little to no deflection;

1 in 2000 were deflected.

Few α particles were deflected more than >90 degrees; 1 in 10,000 were delfected.

7

What did the Rutherford scatterering experiment confirm?

Most of the atom is empty space (as most α particles passed straight through)

A central positively charged nucleus (as some α particles are deflected/repelled by the nucleus)

The mass and charge are concentrated in the nucleus (if not then the α particles would not be deflected backwards).

8

For an α particle with charge, Qα, approaching a nucleaus of charge, QN:

When the initial kinetic energy is equal to the potential energy of the nuclei's electric field.

The alpha particle is at a distance of least approach d.

EK = Qα*QN/4πε0d = +2e*+Ze/4πε0d

9

Why is the probabilty of α particles scattering once = 1 in 10,000n for n layers of atoms.

For each layer added, there are more atoms in the path of the beam, so the probablity of the alpha particle hitting the given nucleus ∝ 1/n.